Rails cars and locomotives are known to be equipped with a service braking system wherein brakes on each individual car are applied by a central air-operated piston and cylinder system. A mechanically applied hand brake is also provided on most rail cars and locomotives for periods when the train is at rest and the air supply to the braking system is inactive, or for when a car or locomotive is parked apart from a train consist.
A typical prior art locomotive hand brake assembly is illustrated in FIG. 1. The hand brake system 10 includes a brake shoe 12 that may be selectively urged against a wheel 14 or axle of the locomotive by the operation of a hand wheel 16 or crank. Wheel 14 may be a drive wheel powered by drive motor 15. The hand wheel 16 is connected to a drive chain 18 though one or more gears 20 so that the chain 18 may be caused to rotate a bell crank 22 to apply the brake shoes 12 against the wheel 14. The drive chain 18 has a brake-pulling side 24 and a slack side 26. Such hand brake systems are available from Ellcon National, Inc. of Greenville, S.C. (www.ellcon.com)
A hand brake is designed to prevent a stationary rail vehicle from moving as a result of any slight grade in the track upon which it is parked or as a result of a momentary bump from an adjacent vehicle. Hand brakes are not designed for dynamic braking of the vehicle. Damage may result If a rail car or locomotive is operated with its hand brake engaged. If the brake system 10 is partially engaged when the vehicle is moved, the build-up of heat between the brake shoe 12 and wheel 14 may be sufficient to cause damage to one or both of these components or the associated axle, bearings, suspension, etc. If the brake system 10 is engaged sufficiently to prevent the wheel 14 from rotating when the locomotive is moved, there will be a build-up of heat between the sliding wheel 14 and the passing rail 28. Here again there may be damage to the wheel 14 and associated suspension parts, including the wearing of a flat area on the wheel 14. Should such an out-of-round wheel 14 subsequently be allowed to roll with the vehicle, there may be further damage caused to the rail 28 or even a derailment of the vehicle.
It is known in the art to provide a remote indication of the position of the hand brake system 10. U.S. Pat. No. 3,854,417 describes an indicator flag mounted on the side of a rail car. The indicator flag is connected to the hand brake wheel so that the position of the flag provides a visual indication of the position of the hand brake system that can be viewed from a position remote from the rail car. The utility of such a system is limited in conditions where visibility is poor, such as when an operator is located at a distant end of a long train, or during periods of low visibility such as in fog or darkness.
U.S. Pat. No. 4,296,707 describes a system that provides a leak path in the service brake air system to indicate when a hand brake is engaged. When the service brake air system is pressurized in preparation for train departure, a low pressure in the service brake system will alert the operator to the existence of an abnormal condition. The leak path may function as a whistle to provide an audible alarm of the engaged hand brake. This system adds a potential failure mechanism to the important service brake air system.
It is also known to provide an alarm or to take other mitigating action when a train is operated above a predetermined speed with a hand brake engaged. U.S. Pat. Nos. 5,394,137 and 5,701,974 describe air-operated parking brake systems. The apparatus of U.S. Pat. No. 5,394,137 senses the presence of operating pressure in an air brake system and provides an alarm if the speed of the train exceeds a predetermined value. The apparatus of U.S. Pat. No. 5,701,974 will function to automatically release an air brake system when the train exceeds a predetermined speed. Most locomotives are also equipped with a locked-axle detection and alarm system that works in conjunction with a wheel slip detection system. These systems will provide an alarm if one axle is turning at a significantly slower speed than the others when the train is operating at above a predetermined speed. Once a locked axle alarm has sounded, administrative procedures require that the train be stopped and inspected. All of these systems serve to mitigate the damage that may be caused by operating a train with a hand brake engaged, but they do not prevent such damaging operation.
It is known to provide a switch 30 to detect the position of the slack side 26 of a drive chain 18 in a mechanical hand brake system 10. The switch 30 completes an alarm circuit 32 including an indicator lamp 34 for warning an operator that the hand brake is engaged. The term “engaged” is used herein to include a range of positions between fully engaged and partially engaged when the brake is not fully released, as selected by the system designer. It is important to notify an operator when a hand brake is providing a degree of braking that may cause damage if the vehicle is propelled. Switch 30 may be calibrated to provide such a signal at whatever degree of engagement is deemed appropriate by the designer of the system. In many embodiments such a signal is provided whenever the hand brake is in a not fully disengaged position. Prior art systems move the switch 30 with a mechanical linkage 36 that is connected to the slack side 26 of chain 18. When the hand wheel 16 is rotated to engage the brake shoe 12 against wheel 14, the slack side 26 is lowered by gear 20 and the linkage 36 depresses the switch 30. The linkage 36 is lifted away from the switch 30 when the brake shoe 12 is disengaged and the slack side 26 is lifted by gear 20. Accordingly, the switch must support the weight of at least a portion of chain 18 when the hand brake system 10 is engaged. Typically, the switch will bear about 20 pounds of weight in this condition in some applications. If the rail vehicle is operated with the brake system 10 engaged, the switch 30 will be subjected to additional dynamic loads caused by the movement of the chain 18 resulting from the motion of the vehicle. Failures of switch 30 are known to have resulted from such dynamic load conditions.